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 A263997 Sequence of block lengths in a block spiral of width 1. 2
 1, 2, 2, 2, 3, 3, 4, 4, 5, 5, 6, 6, 7, 7, 8, 8, 9, 9, 10, 10, 11, 11, 12, 12, 13, 13, 14, 14, 15, 15, 16, 16, 17, 17, 18, 18, 19, 19, 20, 20, 21, 21, 22, 22, 23, 23, 24, 24, 25, 25, 26, 26, 27, 27, 28, 28, 29, 29, 30, 30, 31, 31, 32, 32, 33 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS A left-handed block spiral is created by the following pattern: Start with a 1x1 square [ a(1)=1 ]. Attach a block of length 2 and width 1 (a horizontal 1x2 rectangle) to the upper side of the square [ a(2)=2 ] pointing to the right. Attach to the corner a vertical 2x1 block (rectangle). This block has length 2, so a(3)=2.  Continue with a horizontal 1x2 block, so a(4) = 2. And so on. See the sketch shown in the link. LINKS Hans G. Oberlack, Sketch of a block spiral FORMULA a(n) = a(n-2) + 1 for n>4. O.g.f.: x*(1+x-x^2-x^3+x^4)/((1-x)*(1-x^2)). From A008619. - Wolfdieter Lang, Jan 05 2016 CROSSREFS Essentially the same as A008619. Sequence in context: A102605 A112995 A078452 * A135636 A025795 A219610 Adjacent sequences:  A263994 A263995 A263996 * A263998 A263999 A264000 KEYWORD nonn,easy AUTHOR Hans G. Oberlack, Oct 31 2015 EXTENSIONS Edited. - Wolfdieter Lang, Jan 05 2016 STATUS approved

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